Titanium catalysts are known to be useful in the preparation of alkylene oxide. Processes for the preparation of titanium catalysts are also well known.
U.S. Pat. No. 6,114,552 describes the use of a high surface area silica support in preparing epoxidation catalysts. The support used in the invention of U.S. Pat. No. 6,114,552 has a surface area greater than 1100 m2/g. The epoxidation catalyst is made by impregnation of the support with either a solution of a titanium halide in a non-oxygenated hydrocarbon solvent or a gas stream of titanium tetrachloride, and then calcining it.
Catalyst 1B is the only catalyst made in accordance with the invention of U.S. Pat. No. 6,114,552 which is prepared by impregnation of a silica support with a gas stream of titanium tetrachloride. The surface area of the support used for Catalyst 1B is 1140 m2/g. This silica support is first calcined at 550° C. for 14 hours. The calcined and dried support thus obtained is then charged into a tubular reactor which is inter alia equipped with a heating mantle and a 3-neck round-bottom flask. The catalyst inside the tubular reactor is then heated to 400° C. Once this temperature is reached, water is added into the 3-neck round-bottom flask and this flask is heated with a heating mantle to reflux in order to distill the water through the catalyst bed. A heat gun is used to heat the round-bottom flask to drive any residual water through the bed. That is to say, in order to ensure that all of the water added into the flask is brought into contact with the support inside the tubular reactor. Once all of the water has disappeared from the round-bottom flask, the catalyst bed inside the tubular reactor placed on said flask, is cooled to 300° C. Such water treatment may, at least partially, result in hydrolysis of the support. It is not disclosed in U.S. Pat. No. 6,114,552 at what rate the bed temperature is decreased. Neither is it indicated if, and, if so, for how long the catalyst bed is maintained at 300° C. Therefore, as soon as said temperature is achieved, the silica support which has not been dried after the water treatment step, is subjected to the titanation step. In this titanation step, titanium tetrachloride is transferred to the 3-neck round-bottom flask. This flask is heated to reflux in order to distill the titanium tetrachloride through the catalyst bed. After said titanation the catalyst bed is heated at 700° C. for 0.5 hour resulting in a calcined catalyst. The final catalyst has a loading of 4.9 wt % titanium.
Comparative Catalysts 3A and 3B of U.S. Pat. No. 6,114,552 are also catalysts prepared by impregnation of a silica support with a gas stream of titanium tetrachloride. The surface area of their support is only 300 m2/g. This silica support is first dried at 450° C. for 2 hours. The dried support thus obtained is then placed into a tubular reactor which is inter alia equipped with a heating mantle and a 3-neck round-bottom flask. The catalyst inside the tubular reactor is then heated to 200° C. Once this temperature is reached, titanium tetrachloride is added into the 3-neck round-bottom flask and this flask is heated with a heating mantle to reflux in order to distill the titanium tetrachloride through the catalyst bed. After said titanation the catalyst bed is heated at 600° C. for 2 hours resulting in a calcined catalyst. The final catalyst has a loading of 2.6 wt % titanium.
GB-A-1,148,689 relates to a process for preparing an olefin polymerization catalyst which comprises reacting alumina, silica or alumina-silica with a titanium halide at a temperature of from 300 to 700° C., followed by hydrolyzing and calcining in a hydrogen-containing atmosphere. In Example 10 of GB-A-1,148,689 silica is used as the carrier. This carrier is first calcined at 500° C. for 2 hours. The calcined and dried carrier thus obtained is then reacted with a gas stream containing titanium tetrachloride at a temperature of 375° C. for 2 hours. After completion of the reaction, dry nitrogen only was introduced for an additional ten minutes to purge the remaining titanium tetrachloride from the reactor. It is not indicated in GB-A-1,148,689 which temperature is applied during said purging. However, if said temperature is the reaction temperature of 375° C., then the time period of only 10 minutes is still too short, especially in combination with such relatively low temperature, to effect calcination of the impregnated carrier after having first purged all of the remaining titanium tetrachloride from the reactor. In order to achieve calcination, in general, a calcination time of at least 30 minutes may be required, as in the above-mentioned first calcination step (2 hours) and the second calcination step discussed below (4 hours). After said purging the impregnated carrier is hydrolyzed, dried at a temperature of 120° C. for 2 hours, and finally calcined by heating it at a temperature of 800° C. for 4 hours while passing dry hydrogen from which oxygen had been removed, over it.
EP-A-0,525,503 discloses a method for preparing a heterogeneous catalyst comprised of a support material and at least one catalytically active species. In Example 1 of EP-A-0,525,503 silica is used as the support material. The support is first pretreated for 16 hours at 450° C. in air. The dried support thus obtained is then pretreated for 4 hours in nitrogen at a partial vacuum of 70 mbar. It is not indicated in EP-A-0,525,503 which temperature is applied during said pretreatment with nitrogen. However, since after said latter pretreatment the temperature is increased up to the reaction temperature which is either 175 or 450° C., the temperature applied during said latter pretreatment must be lower than 450° C. Once the reaction temperature is achieved, titanium tetrachloride vapour is introduced for 2 hours into the reaction space using nitrogen gas as the carrier. The titanation reaction is performed at said partial vacuum of 70 mbar, which is in conformity with a preferred embodiment of the invention of EP-A-0,525,503 as discussed at column 5, lines 30-38. According to this preferred embodiment, the reaction between the vapour of the catalyst reagent and the support material takes place at a pressure ranging from 0.1 to 100 mbar. According to EP-A-0,525,503, an advantage of that partial vacuum is that purity of the reaction space can be improved and the diffusion rate increased. Finally, the impregnated silica support obtained in Example 10 of EP-A-0,525,503 is flushed with nitrogen gas for 2 hours at the reaction temperature. The final catalyst has a loading of 3.3 wt % titanium where the reaction temperature is 175° C., and a loading of 2.6 wt % titanium where the reaction temperature is 450° C.
There is a continuous interest in improving the selectivity of titanium catalysts to alkylene oxide. There is furthermore interest in improving the process for the preparation of such catalysts. We found a simple and attractive way to achieve this.